The present invention relates to the preparation of biuret and more particularly is concerned with a pyrolysis process for preparing biuret with high urea conversion.
The preparation of biuret by pyrolysis of urea, as well as other methods, has long been known in the art. Many of these preparations are summarized in an article "Biuret and Related Compounds" published in Chemical Reviews, 56, p. 95-197 (1956). Of the various methods for preparing biuret set forth in this review article it was indicated that although difficulties are present, large scale preparations have been developed based on the pyrolysis of urea.
Olin (U.S. Pat. No. 2,370,065) teaches a process for preparing biuret wherein urea is heated to a temperature above its melting point but below the decomposition temperature of the biuret and by-product ammonia formed by the condensation of urea is swept from the reaction zone with a stream of a hydrocarbon gas. In the practice of the Olin process, the hydrocarbon gas, preferably toluene, is introduced during the reaction period below the surface of the molten urea and the ammonia-hydrocarbon gas mixture rapidly removed from the reaction zone. The ammonia is removed from the resulting hydrocarbon sweep gas-ammonia mixture and the ammonia depleted hydrocarbon gas returned to the reaction zone for removal of further quantities of ammonia.
Harmon (U.S. Pat. No. 2,145,392) teaches a process for preparing biuret by heating urea at a temperature of 130.degree. to 205.degree. C. at a pressure of not substantially greater than 200 mm. of mercury. This allegedly provides a mixture of urea and biuret from which the biuret is subsequently separated.
Kamlet (U.S. Pat. No. 2,768,895) lists a number of references directed to the preparation of biuret by pyrolysis of urea and teaches a process for directly pyrolyzing urea in the absence of a catalyst at a temperature between 120.degree. and 205.degree. C. This effects substantial autocondensation of the urea to produce a mixture of unreacted urea and an admixture of urea autocondensation products, the total mixture consisting of 30 to 70 percent urea with biuret being a predominant component of the autocondensation products. The Kamlet process further includes extracting urea from the resulting product with a selective solvent for urea, e.g. preferably water, to leave a product containing 60 to 90 percent of the admixed autocondensation products with the remainder being urea. The so-extracted product mass is taught to be suitable for use as a protein supplement for ruminant feeds.
Formaini et al. (U.S. Pat. No. 3,057,918) teaches a cyclic process for preparing biuret in which urea is heated at from 135.degree. to 200.degree. C. and the resulting crude pyrolytic product quenched and digested in hot aqueous ammonia until no triuret remains. The liquid mass is then cooled to fractionally crystallize biuret which is removed. The ammonia is stripped from the residual solution whereupon cyanuric acid crystallizes. This solid product is separated from the residual aqueous solution and the solution concentrated by removal of water. The resulting concentrate is recycled with additional urea for subsequent pyrolysis.
Colby (U.S. Pat. No. 2,861,886), Kamlet, (referenced hereinbefore) and other publications attest to the utility of biuret as a feed composition additive for ruminants. This additive provides usable nitrogen to supplement the protein content of feeds from natural sources. Moreover, the use of biuret as a nitrogen supplement is preferred since it is assimilated by ruminant animals at a slower rate than urea-based supplements and consequently avoids the danger of ammonia toxicity to the animal.
While the pyrolysis of urea to yield biuret is well-known in the art, there are many disadvantages associated with such methods. The principal disadvantage lies in the low total conversion of urea to the desired biuret product. Thus, methods wherein urea is directly heated at higher temperatures or for longer periods of time have been employed in attempts to increase the conversion of urea to biuret. It is known, however, that higher reaction temperatures favor the formation of cyanuric acid and a higher total cyanuric acid plus triuret content in the pyrolysis product. Conversely, lower reaction temperatures favor the formation of biuret and a higher biuret content in the pyrolysis product. Moreover, regardless of the prior art direct pyrolysis methods employed, a point is soon reached at which the formed biuret is being converted to higher condensation products faster than more biuret is being formed. Under such conditions, the highest yield of biuret is reached when the pyrolysis product reaches a point at which is contains about 50% biuret. Further heating will give more conversion of urea, but will also cause more of the biuret to become further reacted to form higher condensation products which soon become a solidified mass. Heat transfer through the solidified mass is difficult. If a relatively low temperature (less than 120.degree. C) is employed to avoid higher condensation products, the rate is much too slow to be economically feasible.
In order to thus obtain a ruminant feed product having a high biuret concentration and desirably low concentrations of urea and by-products cyanuric acid and triuret, extraction of the pyrolysis product with water or other solvents to remove a large portion of the urea and byproducts present has thus been necessary in prior art procedures.
In attempting to obviate the problems of the prior art, it was discovered by the applicant and others working with the applicant that the conversion of urea to biuret could be increased by methods wherein the pyrolysis of urea is carried out in certain inert carriers. Such novel methods allow for the close control of the pyrolysis reaction temperature and provide further conversion of urea to biuret while minimizing the build-up of undesired cyanuric acid and triuret by-products. Although such methods provide an improved pyrolysis product containing the desired biuret component in increased concentrations, the applicant and others working with the applicant have discovered that such methods also suffer disadvantages which limit the biuret content of the pyrolysis product that can be obtained to about 55 to about 57 percent. In this respect, as the biuret content of the pyrolysis product is increased and the urea concentration is decreased during the pyrolysis reaction to below 50 percent, the pyrolysis reaction temperature is correspondingly decreased enough to minimize the formation of undesired cyanuric acid and triuret while maintaining the pyrolysis reaction mass in suspension. When the pyrolysis temperature was decreased to about 125.degree. C. and the urea concentration was decreased to below about 40%, usually between about 20 and 40%, it was discovered that the suspended pyrolyzate product would agglomerate, adhering to the reactor walls and agitator blades in sufficient quantities to effectively block further attempts to agitate the reaction mass and maintain the pyrolysis reaction. Much of the solidified reaction mass is lost in difficult recovery efforts from the reactor and the quantity of product that is recovered must be further treated by grinding, milling, etc., procedures to prepare an acceptable product. If the solidified reaction mass is cooled to below about 112.degree. C., further conversion of urea in the mass to biuret can be obtained; however, the rate of conversion is extremely slow and is not economically feasible. The product obtained must also be treated as set forth hereinbefore.
It is a principal object of the present invention to provide a process for production of biuret in high yields by pyrolysis of a urea containing reactant.
It is another object of the present invention to provide a process for production of biuret which assures for good control of reaction conditions and minimizes, if not entirely eliminates, urea reactant losses during the processing.
It is also an object of the present invention to provide a process for preparing biuret by pyrolysis of urea wherein autocondensation by-product formation, particularly cyanuric acid, can be held to a low level.
It is an additional object of the present invention to provide a process which eliminates the need for milling or grinding of the biuret product obtained and provides for the production of biuret in granular or prill form.
It is another object of the present invention to provide a process for preparing a biuret product by pyrolysis of urea in a urea-containing reaction mass wherein there is direct high conversion of urea and a low residual urea level in the product without requiring extraction of such residual urea from the product by a selective solvent, such as water.
These and other objects and advantages of the process of the present invention readily will become apparent from the detailed description presented hereinafter.